It is well known in the art to provide manhole assemblies with steps to facilitate a descent into and ascent from the manhole assembly through the top opening thereof.
One of the preferred techniques is described, for example, in U.S. Pat. No. 3,974,615 issued Aug. 17, 1976, assigned to the assignee of the present invention, which patent teaches the use of hollow plastic manhole step inserts releasably mounted to an inner mold member of a mold assembly. The cast material is poured into the mold assembly and when set, the cast member is withdrawn from the mold assembly. Mounting flanges provided at the open end of the manhole step inserts are arranged to break away from the main body portion of the manhole step insert when the cast member is lifted out of the mold assembly, leaving the manhole step inserts imbedded in the cast material. A pair of such inserts is adapted to receive the free ends of a substantially U-shaped step member, which free ends are force-fitted into the hollow manhole step inserts forming an excellent force fit there between.
The aforementioned U.S. Pat. No. 3,974,615 teaches manhole step inserts having two pairs of flanges arranged at right angles relative to one another. One pair of flanges engages an internal surface of the inner mold member while the other pair of flanges is bent inwardly to pass through a substantially rectangular-shaped opening in the inner mold shell so that the two pairs of flanges cooperate to embrace the inner mold shell therebetween to maintain the manhole step insert in proper position preparatory to pouring the cast material into the mold assembly.
In order to simplify the mounting of the manhole step insert to the inner mold shell and in order to avoid the need for flexing the pair of flanges which are passed through the rectangular shaped opening and which may prematurely break due to said flexing, a design has been developed in which a pair of insertion slots are precision cut into a blank member which is inserted into an opening in the inner mold shell and preferably welded thereto.
FIG. 1a shows a conventional arrangement in which the step insert 10a shown in FIGS. 1b, and 1c comprises a hollow housing having a closed end 10a, four (4) elongated sides 10b through 10e and an open end 10f. A pair of flanges 10g and 10h are integrally joined to the right-hand ends of sides 10b and 10d. A second pair of flanges 10j and 10k are integrally joined to the right-hand ends of sides 10c and 10e. Flanges 10j and 10k extend further to the right than flanges 10g and 10h before bending outwardly.
Rectangular-shaped openings are cut into the inner mold shell 12. One such rectangular shaped opening 12a receives a blank 14 fitted into opening 12a and preferably fixedly secured thereto such as by welding. A pair of slots 15, 16 are precision cut into blank 14 so that the left-hand ends 15b, 16b of slots 15 and 16 have a width greater than the right hand ends 15a, 16a of slots 15 and 16.
The increased width of slot portions 15b and 16b is chosen to be sufficient to permit insertion of the flanges 10j, 10k therethrough. The manner of insertion is such that the step insert 10 is aligned so that the flanges 10j, 10k are moved into alignment with the slot portion 15b, 16b. The step insert 10 is then moved in the direction shown by arrow A to press tabs 10j, 10k through slots 15b, 16b so that they are clear of the slots and extend slightly beyond the concave surface of inner mold shell 12. At this time, the step insert is moved to the right so that the projecting portions 10j-1, 10k-1, are fitted in the narrow slots 15a, 16a and flanges 10j, 10k rest against the concave surface of inner mold shell 12 extending respectfully upwardly and downwardly from narrow slot portion 15a, 16a. Flanges 10g and 10h rest against the convex surface of the blank 14.
Cast material is poured between the inner and outer mold shells 12, 18. When the cast material (typically concrete) is set, the cast member is removed from the mold assembly, typically by lifting the cast member vertically upward. The flanges 10j, 10k break away from the main body of the step insert 10. FIG. 1c shows the jagged edges E where the flanges 10j, 10k have broken away from the main body portion 10. Either a substantially U-shaped aluminum step 20 or plastic polypropylene step 22 is inserted into the step inserts by movement of the free arms 20a, 22a which are pressed or driven into an interference fit within the hollow interior of the step inserts (only half of each of the aluminum (20) and plastic polypropylene (22) step is shown in FIG. 1c for purposes of simplicity.
The insert 10 is preferably made of polypropylene which has a life expectancy exceeding concrete itself and further exhibits excellent chemical resistance. The inserts 10 tend to serve as a protective shield against dissimilar material reaction such as an electrolysis of aluminum and concrete. The insert meets requirements of all ASTM C-478 § 12 and Performance Test Procedures of ASTM C-497.
The insert 10 design shown in FIGS. 1a–1c has a disadvantage that seepage occurs through the larger width slots 15b, 16b provided in the blank 14 while the concrete is being poured into the mold assembly. More specifically, fines were found to seep through the wider slot areas 15b, 16b and enter into the mechanical hinge work provided for release of the cast member. Additional problems occurred when the cured concrete parts stripped out of forms were found to exhibit a honeycomb structure in the region where the fines bled through.
A number of conventional step members utilized in manhole assemblies and the like have been provided with substantially rectangular and in most cases square-shaped, cross-sectional shapes, as can be seen in FIG. 1c. Other step members adapted for insertion into the step inserts are designed to have circular cross-sections. In addition, as can be seen from FIG. 5a, a major portion of the insertable portion of each leg of the step is provided with a generally saw-tooth-shaped configuration wherein the saw-tooth configuration is of such a nature as to facilitate insertion of the legs to the step into each step insert while providing a frictional fit of significantly increased gripping force which acts against a force acting in the direction to withdraw the step legs from the insert.
The steps of circular cross-section thus require a step insert of a design and shape which conforms with and cooperates with the design and shape of the steps whose legs are adapted to be inserted therein.
Manhole assemblies are typically produced through either a drycast or a wetcast method, both of which methods are well known in the art.
Utilizing one conventional drycast method, the granulated casting material is placed into the mold assembly which is vibrated and packed down to assure that the dry casting material is evenly and densely packed within the mold assembly. When the casting material has been filled to the appropriate level within the mold assembly and appropriately compacted, the core portion of the mold extends a pair of reciprocating pins, which are operated by hydraulic means, into the casting material to form openings within the cast material of a size appropriate for receiving the legs of a step. In the conventional technique, the tremendous pressures developed within the casting material cause these openings to “sag” or become “off-round” when the pins are removed and the openings often accumulate some of the cast material when the cast member is removed from the mold assembly which necessitates the operators employing a drill or other device to reopen or “reround” the openings to enable the legs of a step to be inserted therein.
The hydraulically-operated pins which create the step openings typically extend from a core mold member which has a curved, convex outer periphery. The casting material entering into the openings in the region between the pins and the opening in the core member through which the pins extend, serves to render the pins inoperative and possibly even causing damage to the mechanism. At the minimum, the entry of fines into the mechanism require regular maintenance to maintain the mechanism in operating condition, which disadvantages are to be avoided.
In the employment of the wetcast method, there has heretofore been no step insert available which is inserted into the casting material by hydraulically operated pins.